As coated steel sheet with a good corrosion resistance, there is galvannealed steel sheet. This galvannealed steel sheet is usually produced by degreasing the steel sheet, then preheating it in a nonoxidizing furnace, cleaning the surface, securing the desired quality by annealing it by reduction in a reducing furnace, dipping it in a hot-dip zinc bath, controlling the amount of deposition, then alloying it. This is characterized by excellent corrosion resistance and coating adhesion etc., so is being widely used for automotive and building material applications etc.
In particular, in recent years, in the automobile field, higher strength of coated-steel sheet has been considered necessary for securing the function of protecting the passengers against collisions and for reducing weight so as to improve the fuel efficiency.
To increase the strength of steel sheet without detracting from the workability, it is effective to add elements like Si or Mn and P, but addition of these elements delays the alloying, so compared with mild steel, a higher temperature and longer time are required for alloying. This higher temperature, longer alloying causes the austenite remaining in the steel sheet to transform to pearlite and reduces the workability, so as a result the effects of these added elements is canceled out. For the alloying of Si-containing high strength steel sheet, Japanese Unexamined Patent Publication (Kokai) No. 5-279829 discloses a method of production realizable even by a continuous hot-dip galvanizing line, but the range of the production conditions is described extremely broadly and this is of little use in actual production. Further, the method of production disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-131145 causes the formation of retained austenite by holding the sheet at a low temperature after coating, but this invites an increase in the facilities, so detracts from the productivity.